Multi-layer information storage system with improved aberration correction

ABSTRACT

An optical information storage system has a multi-recording layer record carrier and a scanner device for the carrier. The scanner produces a radiation beam which is compensated for spherical aberration for a single height of the scanning spot with the stack of layers. The height of the stack is determined by the maximum spherical aberration permissible for the system. The number of layers in the stack is determined by the minimum distance between layers, which depends on the crosstalk in the error signals due to currently unscanned layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending applicationSer. No. 07/674,493 filed Mar. 25, 1991 by Antonius H. M. Holtslag forMETHOD OF AND APPARATUS FOR OPTICALLY WRITING, READING AND ERASING AMULTI-PLANE RECORD CARRIER, AND RECORD CARRIER SUITABLE FOR SAID METHODAND APPARATUS, assigned to the assignee of the instant application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of optically writing and subsequentreading and/or erasing information in a recording plane of an opticalrecord carrier having at least two recording planes and a guide plane. Aguide beam focused to a spot (guide focus) in the guide plane is usedduring writing, and at least one write beam focused to a writing spot(write focus) in the recording planes, the guide focus and the writefocus being formed by one objective system and the guide focus beingheld in the guide plane by means of a focus error signal generated bythe guide beam.

A multi-layer record carrier comprises a stack of information layersseparated from one another by spacer layers, in which each informationlayer may comprise information. The large information storage capacityof such a record carrier increases its convenience of use compared tosingle-layer record carriers and reduces the price of the medium perunit of information. Each information layer can be scanned independentlyof the other information layers by means of a radiation beam. Dependenton the type of record carrier, information can be written into aninformation layer during scanning and/or information already written canbe read or erased. The information layers in a stack can be scanned bymeans of a radiation beam which is incident from one side on the recordcarrier. For scanning the separate information layers, the height, oraxial position, of the scanning spot formed by the radiation beam isvaried. The information contents of the record carrier may be furtherincreased by implementing the record carrier as a two-sided recordcarrier. Then a stack of information layers is present at both sides ofthe record carrier, and each stack can be scanned from a different sideof the record carrier. A stack of information layers may be provided ona substrate which should be transparent if the stack is scanned throughthe substrate.

2. Description of the Prior Art

A method of the type mentioned in the opening paragraph is described inJapanese Patent Application 63-234418. In accordance with this method anobjective system converges a guide beam to a guide focus on a guideplane in a record carrier. A focus servosystem controls the objectivesystem in such a way that the guide focus remains in the guide plane inspite of possible excursions of the record carrier. A read or writebeam, or generally a scanning beam, is focused by the objective systemon a recording plane to be written or read, which plane is parallel tothe guide plane. For this purpose the read/write focus of a read/writebeam formed by the objective system must be displaceable with respect tothe guide focus in the longitudinal direction, i.e. in the direction ofthe optical axis. Starting from a reference position of the scanningfocus, which reference position is equal to the desired position of theguide focus, this is realised by displacing the radiation sourcesupplying the scanning beam along the optical axis over discretedistances which match the distances between the recording planes.

To be able to use the known method with a so-called passive longitudinaladjustment of the scanning focus, the different recording planes of therecord carrier must be very accurately parallel to the guide planewithin the focus depth of the objective system, because otherwise thescanning focus is not always located in a recording plane to be scanned.A multilayer record carrier having such a high degree of parallelism ofthe layers is difficult to manufacture and is consequently expensive.Moreover, during writing, the scanning focus should accurately follow agiven track in a recording plane to be scanned, while during reading thescanning focus should accurately follow the written information tracks.Japanese Patent Application 63-234418 does not reveal how this so-calledtransversal positioning of the scanning focus must be realised.

An information storage system of the type described in the openingparagraph is known from European Patent Application no. 0 517 491, inwhich a device is described for reading information layers in amulti-layer record carrier. The device is provided with an adjustablespherical aberration compensator for compensating the sphericalaberration incurred by the radiation beam of the device when it passesthrough the material of the record carrier. Since the information layersare located at different heights in the record carrier, the deviceemploys a specific setting of the compensator for each informationlayer. A drawback of this known information storage system is that thereshould be a separate compensation for each information layer. Acompensator which can realise this is complicated and relativelyexpensive. The relatively low cost of the record carrier per unit ofinformation is thus counteracted by a relatively expensive scanningdevice.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method and apparatus of thetype described in the opening paragraph in which a record carrier whichcan easily be manufactured can be used and in which the transversalpositioning problem is also solved.

Another object of the invention to provide an information storage systemhaving a relatively low price and at the same time a high informationdensity.

The Method

In accordance with a first aspect of the invention the method ischaracterized in that during writing the transversal position of thewrite focus in a recording plane is coupled to the transversal positionof the guide focus, the latter position being controlled by a trackingerror signal obtained from the cooperation between the guide beam andthe guide plane, and in that

during reading and/or erasing:

a) a read focus formed by a read beam is held in the scanned recordingplane by means of a focus error signal obtained from the cooperationbetween the read beam and the scanned recording plane,

b) the focusing means introduce such a fixed, stack-associated sphericalaberration in the radiation beam that this aberration compensates thespherical aberration incurred by the radiation beam when it is focusedat approximately half the height of the stack of information layers, and

c) the transversal position of the read focus is controlled by atracking error signal obtained from the cooperation between the readbeam and the scanned recording plane.

The invention is based on the recognition that the transversal positionof the write focus can be controlled by coupling this position to thatof the guide focus by means of guide information in only one plane ofthe record carrier when writing all recording layers, and that the readfocus can be controlled independently of the guide focus when readingwritten recording layers. Further, a stack of information layers can bescanned by a satisfactorily corrected scanning spot, while using asingle, constant and suitably chosen spherical aberration compensation.Since the spherical aberration is not compensated anymore for eachinformation layer individually as in the known system, the focusingmeans can be made simpler, reducing the cost of the scanning device. Thespherical aberration incurred by a focused radiation beam as a functionof the thickness of the material through which the beam passes appearsto be sufficiently small for a reasonably large range of thicknesses,which range is located symmetrically around the thickness for which theradiation beam is well compensated. By compensating the radiation beamin such a way that the scanning spot is substantially free fromspherical aberration at approximately half the height of the stack, itis possible to scan information layers located within said range at bothsides of this half height with a sufficiently low spherical aberration.This provides the possibility of scanning a stack of information layersby means of a scanning beam which is compensated once for sphericalaberration. A device suitable for scanning a record carrier having asingle stack then only needs a single, fixed spherical aberrationcompensation. Since this compensation can be built into a componentwhich is already present in the device, for example an objective lens ofthe focusing means, the construction of the device can be simplifiedconsiderably.

It is to be noted that the abstract of the Japanese Patent Applicationno. 60-202 545 describes an information storage system in which thescanning spot of a radiation beam can be varied in height so as to focuson one of the information layers, located at different heights, of arecord carrier. However, this publication does not describe the problemswhich are caused by spherical aberration due to the different heights ofthe information layers and consequently does not indicate how thisaberration should be compensated for.

Since the spherical aberration due to the traversed material thicknessof the record carrier is dependent on the refractive index of thematerial and on the numerical aperture of the radiation beam, the sizeof the above-mentioned range of sufficiently small spherical aberrationwill also depend on these parameters. Since the height of the outermostinformation layers should be within this range for a correct scanning,the maximum thickness of the stack is preferably a function of therefractive index of the material of the stack and of the numericalaperture of the focusing means.

The size of the range of sufficiently small spherical aberration withinwhich information layers can still be scanned with a sufficient qualityis determined by the admissible deterioration of the quality of thescanning spot, as determined by the scanning device. The deteriorationleads to a less satisfactory detection of electric signals derived fromthe radiation coming from the record carrier. The deterioration which ismaximally admissible for a specific information storage system may beexpressed in terms of the Strehl intensity. The Strehl intensity is thenormalized maximum intensity of the radiation distribution of thescanning spot. If there are no aberrations, the Strehl intensity is 1,and for large aberrations the Strehl intensity goes towards 0. If themaximally admissible decrease of the Strehl intensity due to sphericalaberration is given by r, the size of the range depends on r and themaximum thickness of the stack preferably depends also on r.

The thickness of the stack is preferably smaller than the value 2ddefined by the equation ##EQU1## in which n is the refractive index ofthe spacer layers, λ is the vacuum wavelength of the radiation beam andNA is the numerical aperture of the focusing means. If the informationlayers have such a thickness that the beam is also noticeably influencedby the refractive index of the information layers, the parameter nshould be a weighted average of the refractive indices of the spacerlayers and the information layers instead of the refractive index of thespacer layers. If the refractive indices of the spacer layers and/orthose of the information layers are different, the parameter n should bea weighted average of these different refractive indices.

If the information layers are scanned through a transparent substrate,the spherical aberration caused in this substrate should also becompensated for in the focusing means. Generally, the substrate hassmall thickness variations within a certain thickness tolerance. If thespherical aberration due to a substrate having a nominal thickness iscompensated for, the thickness variations give rise to uncompensatedspherical aberration in the radiation beam. This extra sphericalaberration takes up a part of the above-mentioned maximally admissiblespherical aberration of the information storage system, so that theadmissible spherical aberration incurred in the stack is reduced. Aninformation storage system in which scanning through a substrate isrealised and in which the extra spherical aberration is taken intoaccount is characterized in that the thickness of the stack is smallerthan 2d minus the thickness tolerance of the substrate.

The guide focus is held on a track in the guide plane by a trackingservo. During writing, when there are still no tracks in the recordingplane, the write focus is coupled to the guide focus as regards thetransversal position, i.e. the position in a direction perpendicular tothe optical axis as well as to the tracks. During reading the guidefocus is held on the written tracks by means of an active control in thetransversal direction. The read focus is then also actively focused onthe recording plane to be read.

It is to be noted that Japanese Patent Application 63-298836 describes amethod using a guide beam and a write beam. However, each of these beamsis focused by a separate objective system so that the coupling betweenthe two beams cannot be realised with sufficient accuracy. Moreover, thelatter Patent Application does not describe the writing and reading ofrecord carriers having various recording planes.

If the method according to the invention is further characterized inthat the read beam used during reading or erasing is constituted by theguide beam, it can be implemented with a small number of means and theapparatus for performing the method can be simplified.

For writing a recording plane in a record carrier in which the recordingplanes are constituted by surfaces of separate recording layers whichare separated by spacer layers, the method is preferably characterizedin that the write focus is held in the recording plane by means of afocus error signal which is obtained from the cooperation of the writebeam with the scanned recording plane. The write beam will then remainsatisfactorily focused on the recording plane, even if the guide planeand the recording plane are not parallel within a focus depth.

For writing a recording plane in an unlaminated record carrier themethod is preferably characterized in that the longitudinal position ofthe write focus is guided by the longitudinal position of the guidefocus, the distance between the two positions being determined by theordinal number of the recording plane to be written. Use of this methodleads to a record carrier in which one or more recording planes areformed.

The Apparatus

A second aspect of the invention relates to an apparatus for performingthe method, which apparatus comprises at least one radiation source forsupplying a guide beam and at least one write beam, an objective systemfor focusing the guide beam to a guide focus as well as for focusing thewrite beam to a write focus, and a first servosystem for longitudinallypositioning the guide focus in the guide plane. Such an apparatus isknown from the afore-mentioned Japanese Patent Application 62-68207. Thedrawback of this apparatus is that the read/write beam does not have anyindependent servosystems for longitudinally and transversely positioningthe read/write focus.

Another object of the invention is to provide an apparatus which doesnot have these drawbacks. This apparatus is characterized in that itcomprises a second servosystem for transversely positioning the guidefocus in the guide plane, a coupling of the control of the transversalposition of the write focus to the second servosystem, a read trackingservosystem and a read focus servosystem for transversely andlongitudinally positioning, respectively, a read focus formed by a readbeam, said servosystems using a tracking error signal and a focus errorsignal, respectively, generated by means of the read beam. Sphericalaberration is corrected as described below.

The second servosystem holds the guide beam on the tracks in the guideplane. During writing the transversal position of the write focus iscoupled to that of the guide focus because there is no trackinginformation in an unwritten recording plane. During reading the readfocus must be held on the tracks in the recording plane by its owntracking servosystem. A coupling of the transversal position of the readfocus to that of the guide focus as used during writing cannot be usedduring reading because the transversal positioning of the write focuswith respect to the guide focus during writing cannot be reproduced withsufficient accuracy during reading. For similar reasons the read beammust have its own focus servosystem. To this end an embodiment of theapparatus according to the invention for writing a record carrier inwhich the recording planes are constituted by surfaces of separaterecording layers which are separated by spacer layers is characterizedin that the apparatus comprises a third servosystem for longitudinallypositioning the write focus in a recording plane, using a focus errorsignal which is supplied by the write beam. By virtue of the thirdservosystem the write focus remains in the recording plane, independentof the parallelism of the recording plane and the guide plane.

A further embodiment of the apparatus according to the invention forwriting a recording plane in an unlaminated record carrier ischaracterized in that the apparatus comprises a coupling of the controlof the longitudinal position of the write focus to the firstservosystem. In an unlaminated record carrier a recording plane is notformed until the information is written. Prior to writing, the recordingplane is not present so that a write focus cannot be adjusted. For thisreason the longitudinal position of the write focus must be coupled tothe guide focus during writing.

An embodiment of the apparatus according to the invention may be furthercharacterized in that the third and a fourth servosystem determine thelongitudinal and transversal positions, respectively, of the read focus.Reading and writing can then be effected by means of the same radiationbeam. In this case the apparatus requires only four servosystems for theguide, write and read beams.

A preferred embodiment of the apparatus according to the invention ischaracterized in that the first and second servosystems determine thelongitudinal and transversal positions, respectively, of the read focus.The same radiation beam can now be used as a guide beam and as a readbeam. The apparatus then only requires three servosystems.

To be able to separate the different radiation beams in the apparatus inorder to detect them separately, the beams may have a differentwavelength, a different state of polarization or a different spatialdirection, or a combination thereof.

If the detection systems for the guide beam and read or write beam onthe one hand and the radiation sources on the other hand are located atdifferent sides of the record carrier, the advantage is obtained thatthe power of a radiation beam to be detected is independent of theordinal number of the scanned recording layer.

When using multiple recording plane record carriers, it is necessary todetect a recording plane having a desired ordinal number. When writingand reading a multilayer record carrier it is not sufficient for thescanning beam, or write/read beam, to be exactly focused on a recordingplane, but it is at least as important that the correct, i.e. selectedplane is focused. An apparatus providing this possibility ischaracterized by a recording plane selector which comprises a planediscriminator connected to the output of a focus error detection systemof the scanning beam, a counter connected to said discriminator and acomparison circuit for comparing the counter contents with the ordinalnumber of a recording plane to be scanned.

The plane selection method used in this apparatus differs considerablyfrom and is more reliable than that described in Japanese PatentApplication 62-68207, which works with fixed longitudinal distancesbetween the scanning focus and the guide focus.

The presence of the focus servosystem for the read focus is a previouslymentioned aspect of the invention. The signal supplied by the focuserror detection system comprises information about the presence of arecording plane on or near the scanning focus. When the scanning focusis moved through the recording planes, the plane discriminator canderive a pulse from the above-mentioned signal at any instant when arecording plane passes through the scanning focus. With reference tothese pulses and the direction of movement of the scanning focus thecounter determines the ordinal number of the recording plane passing thescanning focus. In this way it is possible to focus the scanning beam onany desired layer. The plane selection method according to the inventionis applicable to all apparatuses for writing, reading or erasingmultilayer optical record carriers which actively focus on the differentrecording planes in the record carrier.

The second aspect of the invention includes provision for correction ofoptical aberrations in the scanning beam for the different longitudinalpositions of the scanning focus in the record carrier. The thickness ofthe record carrier traversed by the scanning beam, from the outsidesurface to the scanning focus, is dependent on the ordinal number of therecording plane to be scanned. This variable thickness introduces avariable quantity of spherical aberration in the scanning beam, whichspherical aberration detrimentally influences the shape of the scanningfocus. In the case of thickness variations of more than approximately100 μm the scanning beam must be corrected so as to maintain asatisfactory quality of the scanning focus. To this end an apparatus inaccordance with this aspect of the invention is characterized in thatthe apparatus includes at least one spherical aberration corrector foran adjustable correction of spherical aberration in the scanning beam,the magnitude of the correction being dependent on the refractive indexand the thickness of the material of the record carrier in the opticalpath of the scanning beam between the objective system and the scannedrecording layer. A particular embodiment of such an apparatus ischaracterized in that the spherical aberration corrector having anadjustable correction is a transparent plate comprising a plurality ofareas of different thicknesses, each time one of said areas beingpresent in the path of the scanning beam. Plane-parallel plates of, forexample, glass or plastics material can easily be used to correct thespherical aberration for any recording plane to be scanned.

It is to be noted that U.S. Pat. No. 3,999,009 also describes anapparatus for scanning a multilayer record carrier, which apparatus hasa transparent plate which can be introduced into the scanning beam. Thisplate is intended to move the scanning focus longitudinally, with theobjective system being stationary. In contrast to the plate according tothe invention, the known plate does not correct the sphericalaberration, but aggravates it. A further difference between the twoplates is that the known plate should become thicker to move thescanning focus away from the objective system, and that the plateaccording to the invention should become thinner.

The spherical aberration corrector may be generally used in an apparatusfor scanning multilayer record carriers, not only in an apparatus usinga guide beam and a scanning beam, but also in an apparatus without aguide beam.

The invention also relates to a device for scanning information layersof an optical record carrier, which device is provided with a radiationsource, focusing means for selectively focusing a radiation beam fromthe radiation source on separate information layers, and a focusservosystem comprising a focus detection system having aradiation-sensitive surface. Currently unscanned information layersyield a relatively large defocused, parasitic radiation spot on theradiation-sensitive surface. To minimize the crosstalk due to thisparasitic spot on the focus error signal, the radiation-sensitivesurface should be small. On the other hand, the radiation-sensitivesurface should have a certain minimum dimension to enable it to generatea satisfactory focus error signal. According to the invention, thedevice is therefore characterized in that the radiation-sensitivesurface has a largest dimension ranging between 1.5 and 3 times thediameter of the radiation spot formed on the radiation-sensitive surfacewhen the radiation beam is optimally focused on the information layer tobe scanned. The above-mentioned 8 μm peak-to-peak distance of the Scurve can be realised with such a focus detection system. The maximumdimension of the radiation-sensitive surface is preferably approximatelyequal to twice the diameter of said radiation spot. It is possible touse a detection system with a relatively large radiation-sensitivesurface while meeting the above dimensional requirement by arranging adiaphragm in the radiation beam, restricting the extent of the area ofthe radiation-sensitive surface on which radiation is incident. Thereduction of the crosstalk between the focus error signals makes itpossible to reduce the minimum distance between the information layers.

Generally, a device for scanning record carriers is provided with atracking servosystem for causing the scanning spot of the radiation beamto follow information layer tracks in which the stored information isarranged. Such a servosystem will also be influenced by crosstalk due tocurrently unscanned information layers situated proximate to thescanning spot. To minimize this influence, the radiation-sensitivesurface of the tracking detection system preferably has a dimensionranging between 1 and 3 times the diameter of the radiation spot on thetracking detection system when the radiation beam is optimally focusedon one of the information layers. Such a device is particularly suitablefor integration in a storage system according to the first and secondaspect of the invention.

Some types of the device are provided with dividing means located in theoptical path between the radiation source and the focusing means forgenerating two servobeams and one main beam from the radiation beam, thetwo servobeams being used for generating a tracking error signal.According to the invention, crosstalk of tracking error signals can bereduced in such a device by choosing the power in the main beam to besmaller than six times, and preferably 4 times, the power in each of theservobeams. The crosstalk appears to be caused by parasitic radiation ofthe main beam reflected by an information layer which is not to bescanned currently and is incident on the servodetection system. In thedevice according to the invention the amount of radiation in theservobeams is larger than the amount of parasitic radiation of the mainbeam at the location of the radiation-sensitive detectors of thetracking detection system. This reduces the influence of the parasiticradiation and thus the crosstalk, thereby allowing a smaller spacerthickness.

The Record Carrier

The method and apparatus according to the invention provide thepossibility of using a record carrier which cannot only be written in awell-defined manner but also be read satisfactorily. This record carrieris characterized in that the record carrier has a recording layer ofsuch a thickness that it can be provided with different recording planeswhich can be scanned separately. Such a thick recording layer can bemade at a lower cost than a stack of recording layers and intermediatelayers. The recording planes are not formed until the thick recordinglayer is written.

A preferred embodiment of the record carrier is characterized in thatthe guide plane has an inscribable layer. The number of recording planesin the recording layer is extended by one by providing the guide planewith a sensitive layer.

A further preferred embodiment of the record carrier is characterized inthat the guide plane comprises non-erasable information which has beenprerecorded during the production of the record carrier. Thenon-erasable information enhances the facilities of use of the recordcarrier and provides, for example, the possibility of distributingstandard data or programs. The non-erasable information can be laid downin the guide plane simultaneously with the tracking information by meansof a stamping process, as described in, for example GB PatentApplication 2,036,410.

The information in the recording planes is preferably coded inaccordance with a self-clocking recording code. Since the recordingplanes do not comprise any synchronizing marks provided duringproduction, the clock for decoding the signal which has been read fromthe written information in the recording planes, must be generated bythe apparatus itself from said signal.

Still another aspect of the invention is related to rendering theinformation contents of a record carrier as large as possible bymaximizing the number of information layers. However, the number ofinformation layers fitting in a stack is limited by the maximumthickness of the stack on the one hand and by the required minimalmutual distance of the information layers on the other hand. The minimummutual distance is determined by crosstalk of the information layers,i.e. the quality of signals generated from radiation coming from a layerto be scanned is detrimentally influenced by radiation coming from otherinformation layers. The extent of influence is dependent on the type ofsignal. The minimum distance between information layers for anacceptable crosstalk between the information signals of the differentlayers is known from European Patent Application no. 0 605 924 which isherein incorporated by reference. Notably, servo-error signal crosstalkgives rise to additional requirements for the minimum distance. In afocus servosystem, with which the device keeps the scanning spot on theinformation layer to be scanned, the shape of the S-curve, i.e. theresponse curve of the focus servosystem as a function of the distancebetween the scanning spot and the information layer, is influenced by aneighbouring information layer. The S-curve generally has a positive anda negative extreme, while the zero crossing in between is the pointtowards which the servosystem controls the position of the scanningspot. In accordance with the invention, the thickness of each spacerlayer in the stack ranges between 1.5 and 4 times the refractive indexof the spacer layer multiplied by the peak-to-peak distance of theS-curve of the focus servosystem. A thickness below said range givesrise to a large crosstalk, whereas a thickness above said range gives anunnecessary decrease of information density of the record carrier. Theheight is preferably approximately twice the refractive index of thespacer layer multiplied by the peak-to-peak distance. The minimum heightof a spacer layer is approximately 18 n μm for a focus servosystem witha peak-to-peak distance of 12 μm. A special focus servosystem designedfor scanning multi-layer record carriers has a peak-to-peak distance of8 μm, so that the minimum thickness of the spacer layer is 12 n μm.Although the features of the second aspect of the invention can beadvantageously implemented in an information storage system with thespecific correction of the spherical aberration according to the firstaspect of the invention, the application is not limited thereto.

The invention further relates to an optical record carrier having aplurality of information layers at different heights in the recordcarrier, which information layers are separated by spacer layers, whichrecord carrier is suitable to be read by means of a focused radiationbeam employing a fixed spherical aberration compensation. According tothe invention, the record carrier is characterized in that the distancebetween the highest and lowest information layer is smaller than a value2d defined by ##EQU2## in which n is the refractive index of the spacerlayers, λ is the vacuum wavelength of the focused radiation beam, NA isthe numerical aperture of the focused radiation beam and r is 0.05. Thenumerical aperture of a beam is equal to the sine of the half apex angleof the beam in vacuo. Such a record carrier can be scanned by arelatively cheap scanning device, thereby reducing the cost of theinformation storage system.

A system in which stringent requirements are imposed on the quality ofthe scanning spot requires a record carrier in which the height of thestack is smaller than has been indicated in the previous paragraph.According to the invention, such a record carrier is characterized inthat the value of r is 0.01.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a shows an embodiment of a plane selector,

FIG. 1b shows an embodiment of a first apparatus according to theinvention,

FIGS. 2a, 2b and 2c show embodiments of a record carrier having separaterecording layers,

FIG. 3 shows a record carrier having an unlaminated recording volume foruse in the apparatus,

FIGS. 4a and 4b show embodiments of a longitudinal shifter,

FIGS. 4c and 4d show embodiments of a transversal shifter,

FIG. 5 shows a part of an embodiment of the apparatus for reading intransmission,

FIG. 6 shows a simplified optical information storage system accordingto the invention, comprising a record carrier and a scanning device;

FIG. 7 shows the detection systems for the device of FIG. 6;

FIG. 8 shows the position of three spots on an information layer;

FIG. 9 shows the focus error signal as a function of the axialdisplacement of the scanning spot, and

FIG. 10 shows a record carrier scanned through a substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1b shows a part of an optical record carrier 1 in a cross-section.Therecord carrier has a reflecting surface defining a guide plane 2provided with guide tracks 3 which are perpendicular to the plane of thedrawing. These guide tracks are provided in the guide plane during themanufacture of the record carrier and may consist of, for example,continuous grooves or of series of pits in the guide plane. The recordcarrier also comprisesvarious recording planes 4 one of which is shownin the drawing, intended for recording (user) data. The guide tracks ofthe guide plane are not copied in the recording planes.

The apparatus for writing and reading information, for example, data inthis record carrier uses two radiation beams, a guide beam 5 and ascanning beam 6. The guide beam 5 is shown in the drawing by means ofsolid lines and the scanning beam 6 is shown by means of broken lines.In parts of the optical path where the two beams may coincide, the solidlineand the broken line are shown juxtaposed, just to indicate thatthere are two beams.

The guide beam is generated by a radiation source 5' arranged in thefocus of a lens 5" and passes to an objective system 10 via a mirror 9.This system focuses the guide beam on the guide plane 2 of the recordcarrier. In order to keep the guide focus 11 of the guide beam in theguide plane when the record carrier is moving, the longitudinal positionof the focus,i.e. the position along the principal axis of the beam,must be actively controlled. To this end radiation reflected by theguide plane and captured by the objective system is passed from theguide beam to a first detection system 13 via a beam-separating element7, for example, a partially transparent mirror, and a lens 13'. Theoutput of this system supplies a focus error signal Sf, i.e. a signalwhich is representative ofthe distance between the guide plane and theplane in which the guide beam is focused by the objective system. Thefocus error signal controls a linear motor 15 via a switch 30 and afirst servo amplifier 14, which motor determines the longitudinalposition of the objective system 10 and hence that of the guide focus11. The detection system 13, the amplifier 14, the motor 15 and theobjective system 10 jointly constitute the first servosystem of theapparatus. This first servosystem ensures that the guide beam 5 isalways exactly focused on the guide plane 2. This is necessary to ensureoptimum detection in the guide information present in this plane sothat, inter alia, the guide focus 11 always follows a desired track 3 inthe guide plane.

This tracking is possible because the detection system 13, whichcaptures the guide beam radiation reflected by the guide plane, alsosupplies a so-called tracking error signal Sr. This signal isrepresentative of the extent to which the centre of the guide focuscoincides with the centre line of a track to be followed in the guideplane. The signal Sr controls a pivot-drive member 17 for the pivotedmirror 9 via a servo amplifier 16.By pivoting this mirror about an axisperpendicular to the plane of the drawing, the guide focus can bedisplaced in the transversal direction, i.e. in a direction in the guideplane and transverse to the track direction. The detection system 13,the amplifier 16 and the drive member 17 with the mirror 9 constitute asecond servosystem.

The scanning beam is generated by a second radiation source 18 arrangedin the focus of a lens 18'. This beam is coupled by a reflecting element19 to a coupling element 8, for example, a partially transparent mirroror a wavelength-selective mirror in the path of the guide beam 5. Viathe reflector 9 this beam reaches the objective system 10 which forms ascanning focus 36. This scanning focus must always be positionedaccurately, both in the longitudinal direction (parallel to the beamaxis)with respect to a recording layer 4 to be scanned, and in thetransversal direction in this layer.

As regards the tracking information or, more in general, the guideinformation, a record carrier having different recording layers may inprinciple be implemented in different ways. FIG. 2a shows a recordcarrierhaving a guide plane 2 which can be supported by a substrate 40.The guide plane comprises guide tracks 3 in the form of continuousgrooves or consisting of discrete pits having a depth of the order of 60rim. The recording layers 4₁ to 4_(n) are preferably 10 to 100 nm thickand are separated by means of spacer layers 42. If these spacer layersare considerably thinner than 1 μm, the contours of the guide tracks 3,which in principle are only provided in the guide plane, will also bepresent in the first recording layer 4₁, the second recording layer 4₂,and so forth, but with a strongly decreasing depth and will therefore beless suitable for tracking. Said contours are not present in the higherinformation layers.

Another possibility is to provide each recording layer separately withguide information, as is shown in FIG. 2b. However, for each recordinglayer a replication process must be performed with the aid of a stamp,which renders the manufacture of the record carrier very expensive.

According to the invention use is made of a record carrier shown in FIG.2awhose guide plane only is provided with guide information. Wheninformationis being written in a recording layer, use is made of thisguide information which is detected and followed with the guide beam.

As already indicated hereinbefore and shown in FIG. 1b, this is realisedinthat the write beam is coupled into the path of the guide beam so thatthe write beam is passed via the pivotal mirror 9 for guiding in thetransversal direction simultaneously with the guide beam. In principle,the write focus then follows the same track as the guide focus exceptfor being offset longitudinally, while the second servosystem (13, 16,17, 9) enables the guide focus to follow the guide tracks 3 veryaccurately, for example, within 100 nm.

This passive transversal control of the scanning focus is satisfactorilyusable during the writing operation because in this operation it is onlyimportant for the write focus to follow the same or similar track as theguide focus. It is then unnecessary for the write focus track projectedinthe guide plane to coincide exactly with that of the guide focus. Thiswould be different if a recording plane which has already been writtenwere read by means of a read focus whose tranversal position would becoupled to that of the guide focus in the manner described hereinbefore.In this coupling the mutual position of the scanning focus and the guidefocus are determined by the mutual position of the two radiation sources5' and 18 and by the position of the beam splitters 7, 8 and 19. If theoptical system between the radiation source and the focus has amagnification factor of, for example 5, the mutual distance between theradiation sources should be kept constant within, for example, 500 nm soas to follow the tracks in the recording plane within 100 nm. Due tomechanical instabilities and thermal effects, such tolerances are verydifficult to realise in an apparatus.

If a record carrier is to be written with a first apparatus and readwith acorresponding second apparatus, the reading operation by means ofa read beam and a guide beam poses the additional problem that thedeviation between the positions of the guide focus and the read focus inone apparatus is different from that in the other apparatus.

To prevent said stringent tolerance requirements and problems, thetransversal position of the scanning focus is actively controlledaccording to the invention during the reading operation by means of readbeam radiation reflected by a scanned recording plane. This radiationfollows the path of the read beam in the reverse direction and iscapturedby a detection system 22 via the partially transparent element19 and a lens 22'. The transversal control of the read focus isperformed by a third servosystem comprising the detection system 22, afirst switch 23, aservo amplifier 24 and a transversal shifter 25. Thedetection system 22 supplies a tracking error signal S_(r1) whichrepresents the transversaldistance between the scanning focus and thecentre of a track in a recording plane. When a recording plane 4 isbeing written, the switch 23 is open and there is no active control forthe tracking of the scanning focus. During reading the switch is closedand the tracking error signal S_(r1) is passed on to the amplifier 24which in its turn applies the amplified signal to the transversalshifter 25. The shifter is an optical element which can change thedirection of the scanning beam through a small angle. The objectivesystem converts this change of direction into achange of the transversalposition of the scanning focus. The tracking control of this thirdservosystem may be superimposed on the control of the second servosystemwhich operates via mirror 9.

However, it is preferable to switch off the second servosystem duringreading and to cause the servo amplifier 24 to control the mirror 9instead of the transversal shifter 25. The latter is then superfluous.

If the recording layers have a thickness of 300 to 500 nm and the spacerlayers have a thickness of 0.5 to 1 μm, as proposed in Japanese PatentApplication 63-234 418, there is the problem that, if the scanning beamisfocused on one of the recording planes, a relatively small radiationspot is formed at the area of the adjacent recording planes due to thenot infinitely small depth of focus of the beam. The depth of focus of abeam having a numerical aperture of 0.52 and a wavelength of 0.82 μm is±1.5 μm, which means that the intensity on the optical axis, at a point1.5 μm away from the focal point, is still a factor of 0.8 times that inthe focal point. When a recording plane is being written, an adjacentplane at a distance of 1 μm will also be written, while the adjacentplanes will produce strong interference signals when a recording planeis being read.

This problem could be solved by rendering the recording layerswavelength-selective and by placing a separate radiation source in theapparatus for each layer. Then a scanning beam for a given recordingplanewill not influence other recording planes or will not beinfluenced. A drawback of this solution is the limited choice ofmaterials for the recording planes and of radiation sources havingdifferent wavelengths. This considerably limits the possible number ofrecording planes in the record carrier.

A better method is to render the thickness of the spacer layers 42considerably larger than the depth of focus of the beam. However, thisrequires a production method which is different from that used for thinlayers. Layers up to a thickness of 1 μm can be made by sputtering orvapour deposition. However, these processes are too time-consuming forthicker layers. Spin-coating is better for this purpose. The currenttechnology does not provide the possibility of maintaining the thicknessvariation of spin-coated layers well within 1 μm.

Due to these thickness variations a passive longitudinal control of thescanning focus, as proposed in Japanese Patent Application 63-298836,can no longer be used. In fact, in this control the scanning focus islongitudinally placed at discrete distances from the guide focus, whichdistances are determined by the ordinal number of the recording layer tobe scanned. In this case the distances between the guide plane 2 and therecording planes 4, and hence the thickness of the spacer layers 42 areassumed to be very accurately constant. If the variation of thethickness of the spacer layer is larger than the depth of focus of thescanning beam, the scanning focus will not always be located in therecording planeto be scanned, even if the guide focus is located in theguide plane.

According to the present invention this problem is solved by activelycontrolling the scanning focus during writing as well as during reading.Use is then made again of scanning beam radiation reflected by therecording layer to be scanned, which radiation is captured by thedetection system 22. This system supplies a focus error signal S_(f1)which comprises information about a deviation between the longitudinalposition of the scanning focus and the recording plane to be scanned.The active longitudinal control of the scanning focus is performed bymeans ofa fourth servosystem comprising the detection system 22, arecording plane selector 26, a servo amplifier 27, a second switch 28and a longitudinal shifter 20. The recording plane selector 26 in FIG.1a comprises a plane discriminator 26₁ which supplies a pulse for eachpassage of the scanning focus through a recording plane. Use can then bemade of, for example, the focus error signal which has a zero crossingat each recording plane and guide plane. If the scanning focus scans thelayer packet of the record carrier, the plane discriminator will supplya pulse upon the passage of each recording plane. A counter 26₂ countsthe pulses, while the direction of movement of the scanning focus withrespectto the recording planes determines whether additions orsubtractions must be carried out. The output of counter 26₂ is connectedto a first input of a comparison circuit 26₃, while the ordinal numberof the selected recording layer is applied to a second input. At theinstant whenthe desired recording plane passes, the focus error signalS_(f1) is passed on to the servo amplifier 27 via a switch 26₄ which isoperated by the comparison circuit 26₃. The output signal of thisamplifier is applied to the longitudinal shifter 20 via a closed switch28, so that the scanning focus is made to coincide with the selectedrecording plane. The focus control of this fourth servosystem issuperimposed on the control of the first servosystem operating viaobjective 10. It is now possible to actively keep both the guide focusandthe scanning focus in the correct plane.

Correction of Aberration

When a focused beam passes through a plate of transparent material, theplate will generate a quantity of spherical aberration in the beam,proportional to the thickness of the plate. The spherical aberration hasadetrimental influence on the quality of the beam focus. The scanningfocus 36 can be positioned on different layers in the record carrier 1.When thescanning focus is longitudinally shifted through the recordcarrier, the thickness of the record carrier material to be scannedbetween the objective system 10 and the scanning focus 36 will change.As a result, the quantity of spherical aberration at the location of thescanning focuschanges. Apparatuses comprising an objective system 10having a numerical aperture of 0.52 can generally allow a thicknessvariation of + or -50 μm without the scanning focus becoming too bad. Inother words, the scanning focus has a depth range of 100 μm. If therecording planes have a mutual distance of approximately 15 μm, i.e.several times the depth of focus of the objective system, only a fewrecording planes can beprovided in a thickness of 100 μm. If more ofsuch recording planes are to be provided in a record carrier, thescanning focus will have to shift more than 100 82 m. Then a correctorwill be required to correct the generated spherical aberration in thescanning beam. A correction for every 50 or 100 μm of longitudinaldisplacement of the scanning focus is generally sufficient.

A simple corrector comprises a plate of glass or synthetic materialwhich can be placed in an uncollimated part of the scanning beam. Thethickness and refractive index of the plate should be such that thequantity of spherical aberration in the scanning beam required for thecorrection is generated. The thickness and refractive index aredependent on the vergence of the scanning beam at the area of the plateand at the area of the record carrier.

A correction for spherical aberration in the apparatus of FIG. 1b can beperformed by placing two correctors in the guide beam as well as in thescanning beam. A corrector C1, for example, a plate having areas ofdifferent thicknesses is arranged in the scanning beam 6 between theradiation source 18 and the collimator lens 18'. The scanning beam canscan any of these areas by displacing the corrector. If the thinnestpart of the corrector is present in the scanning beam and the scanningfocus 36is positioned on the recording layer 4 which is farthest remotefrom the objective system 10, the scanning focus should be substantiallyfree from spherical aberration. This should be realised with a speciallens design of the objective system. If the scanning focus is positionedon a recording layer closer to the objective system, a thicker part ofthe corrector C1 must be positioned in the scanning beam so as to obtainthe desired correction of the spherical aberration. If it is necessaryto havea minimum aberration at the area of the detection system 22, asecond corrector C2, which is comparable to corrector C1, should also bearrangedbetween said detection system and the lens 22'. The correctorsC1 and C2 should always generate an equal quantity of sphericalaberration in the scanning beam. A third corrector C3 between theradiation source 5' and the collimator lens 5" should ensure that, afterpassage through the objective system having said special design, theguide beam 5 supplies a scanning focus 11 on the guide plane 2, whichfocus is free from aberrations. The same corrector C4 can be arranged infront of the detection system 13 for the same reason as the corrector C2is arranged infront of the detection system 22. The correctors C3 and C4can be integrated with the lens 5" and the lens 13', respectively, bygiving these lenses a modified design. The spherical aberrationgenerated by the correctors C1 and C3 in the scanning and guide beamscauses the beams to flare out between the correctors and the objectivesystem. The detrimentaleffects can be mitigated by shortening theoptical paths of the guide and scanning beams as much as possible.

The correctors C1 and C2 may alternatively comprise a plurality ofplates each having a constant thickness, one or more of which may bepresent in the scanning beam.

An advantageous embodiment of the apparatus has only one corrector C5 inthe longitudinal shifter 20 to be described, instead of the twocorrectorsC1 and C2 arranged in from of the radiation source 18 and thedetection system 22. As is shown in FIG. 4a, the construction of thecorrector C5 can be compared with that of the corrector C1.

A further simplification is obtained if the guide plane 2 in the recordcarrier 1 is not the plane located closest to the objective system 10,as is shown in FIG. 1b, but if it is the farthest remote plane. Thecorrectors C3 and C4 can then be dispensed with.

In a special embodiment the scanning apparatus using a guide beam and ascanning beam has only one corrector which is arranged between theobjective lens 10 and the record carrier 1. The corrector influences thetwo beams at this location. The guide focus is then allowed to receive acertain quantity of spherical aberration from the corrector. Thiscorrector increases the depth range of the scanning focus withoutcorrection at least by a factor of two.

In a multilayer plate scanning apparatus without a guide beam theaberration correction can also be performed by one adjustable correctorwhich is arranged between the objective lens and the record carrier.

The user data may be written in any form in the apparatus, dependent onthetype of sensitive material in the recording plane: in the form ofmagnetic domains, in alloy phase-varied areas, in crystallisationstate-varied areas, etc. The data in the recording planes can be read bymeans of the detection system 22 in the reflected scanning beam, whichsupplies an information signal S_(i1) which is applied to a processingunit 32. As long as the detection system can read the data in arecording plane, thereis certainly a signal which is strong enough togenerate a tracking error and focus error signal for the control of theservosystems of the scanningbeam. A reflection coefficient of a fewpercent of a recording plane is found to be sufficient already.

When reading information, a clock signal for correctly decoding the readsignal must be generated in the processing unit 32. Each recording planecould be provided with synchronization marks from which the clock signalcan be derived. However, if this is done during manufacture, anexpensive stamping or replica from process is required for eachrecording layer. Therecord carrier therefore preferably comprisesspin-coated layers without any synchronization marks stamped into them.In such a record carrier the clock signal must be generated from theinformation written into the recording planes. It is thereforerecommended to write the data into the recording layer with aself-clocking code. The processing unit itself can then derive the clocksignal from the information signal S_(i1). An example of such a code isthe (2,7) recording code known from U.S. Pat. No. 3,689,899.

A novel embodiment of a record carrier which can be used in theapparatus described is shown in FIG. 3. The single recording layer 43has such a thickness that various recording planes 4 can be written intoit. FIG. 3 shows two recording planes. Before writing, the recordingplane is still undefined. Therefore, the scanning focus with which theplane is written should not only be coupled to the guide focus in thetransversal directionbut also in the longitudinal direction, which guidefocus follows the tracks in the guide plane 2. During writing the switch23 is open and the longitudinal position of the write focus isdetermined by an adjusting member 21 which is then connected to thelongitudinal shifter 20 via the switch 28. This adjusting membersupplies a signal which may have a numberof discrete levels eachcorresponding to a given longitudinal position of the write focus in therecording layer 43 of the record carrier shown in FIG. 3. After a givenplane 4 of this record carrier has been written, it may serve as a planefor the active focus control. The switch 28 is then in a position inwhich the output of the servo amplifier 27 is connected to thelongitudinal shifter 20. This control is used when reading a writtenlayer 4, while switch 23 is closed.

In a further embodiment of the record carrier the guide plane can alsobe provided with a sensitive layer so that this plane can also beprovided with user data, thus increasing the storage capacity of therecord carrier.

If various users want to have a quantity of data available which is thesame for all of them (standard data) in addition to their own specificdata, the manufacturer can prerecord this standard data on the recordcarrier, preferably in the guide plane.

Some aspects of the apparatus will be further described. FIGS. 4a, 4b,4c and 4d show several embodiments of the shifters 20 and 25. Thelongitudinal shifter of FIG. 4a has two lenses 50 and 51 producing anapproximately collimated beam 6 from the radiation emitted by the source18. The vergence of the outgoing beams can be slightly changed by asmall displacement of lens 51 along the optical axis. As a result, thefocus formed by the objective 10 is displaced in the longitudinaldirection. When using a focus motor, which is used in a known CD playerfor displacing lens 51, the scanning focus 36 can be positioned on adifferentrecording layer within a few milliseconds. The plate C5 can bearranged between the lenses 50 and 51 for correcting sphericalaberration. Another embodiment of a longitudinal shifter, known fromJapanese Patent Application 63-234418 is shown in FIG. 4b. The radiationfrom the source 18 is formed to an approximately collimated beam 6 by acollimator lens 52. The source 18 is arranged on a piezoelectric crystal53. The laser canbe displaced along the optical axis over a smalldistance by means of a voltage across the crystal. The vergence of theoutgoing beam can thereby be varied.

The transversal shifter of FIG. 4c comprises a folding mirror 54 whichis arranged in the scanning beam 6. A rotation of the mirror changes thedirection of the beam 6, which change of direction is converted by theobjective into a transversal shift of the scanning focus 36. Thesub-plate19 can be used as a folding mirror if the radiation source 18and the detection plane 22 are interchanged. The direction of the beam 6can also be changed by means of an acousto-optical modulator 55, as isshown in FIG. 4d. The change of direction of the outgoing beam 6 isdependent on the control voltage 56 which is applied to the modulator.

The apparatus according to the invention may alternatively be formedwithout the transversal shifter. Instead of a separate scanning beam 6,guide beam 5 is then used for reading the recording planes. Duringwritingthe scanning beam is used together with the guide beam, while anactive transversal control of the scanning beam is not necessary.

For a satisfactory operation of the apparatus it is desirable tocalibrate the longitudinal and transversal shifters 20 and 25. When anunlaminated record carrier is being written, the longitudinal positionof the write focus is determined by the adjusting member 21 and thelongitudinal shifter 20. The inevitable variation of parameters in thesecomponents necessitates a calibration if the components are used in anon-feedback system. For the purpose of calibration the guide focus ispositioned in the guide plane by means of the lens 10. Subsequently, theadjusting member 21 is adjusted in such a way that the write focus isalso located in the guide plane. This can be checked by comparing theinformation in the signal S_(i1) of the detector 22 and in the signalS_(i) of the detector 13. Based on this calibrated adjustment, thelongitudinal position of the write focus can now be changed in smallsteps for writing the different recording planes.

A comparable calibration of the transversal shifter 25 is to berecommendedbefore writing in a recording plane. This is particularlydesirable if the same recording plane is further to be written behind apreviously written area in this recording plane. It is most likely thatthe transversal distance between the guide focus and the write focus atthe start of the second writing action will no longer be the same as atthe end of the previous writing action, which is due to adjustmentvariations in the apparatus. This presents the risk that the tracks tobe written pass through the last-written tracks of the previous writingaction. This can be avoided by means of a calibration. To this end theguide focus and the write focus are positioned in the guide plane, asdescribed hereinbefore. Subsequently, the transversal shifter 25 isadjusted by means of an adjusting member (not shown in FIG. 1) so thatthe write focus is on the same track as the guide focus. This can bechecked by comparing the information signals S_(i) and S_(i1) of theguide beam 5 and the beam 6, respectively. Subsequently the write focuscan be moved to a recording plane to be written while maintaining theadjustment of the transversal shifter.

FIG. 5 shows a part of an embodiment of the apparatus in which therecord carrier is read in transmission. The scanning beam 6, shown inbroken lines in the drawing, now scans all layers of the record carrier1 so thatthe power of the beam to be detected is independent of therecording plane on which the scanning focus is positioned. A lens 61focuses the beam which has passed the record carrier on a detector 62,which then supplies the information signal S_(i1). If the scanning beam6 as well as the guide beam 5 are present during reading, a filter 63 isused which passes only one of the beams to the detector 62 so as toavoid disturbance of theinformation signal.

Such a separation between the scanning beam and the guide beam is alsonecessary elsewhere in the apparatus to ensure that the detectionsystems 13 and 22 in FIG. 1b only receive radiation from the beams 5 and6, respectively. To this end two filters 34 and 35 are arranged aroundbeam splitter 8. The properties of the filters depend on the way of beamseparation. When radiation sources of different wavelengths are used, 34and 35 will be chromatic filters. When different states of polarizationofbeams 5 and 6 are used, 34 and 35 will be polarization filters. Suchfilters can be satisfactorily combined to a single element with the beamsplitter 8 in the form of a cube on which chromatic filters have beenvapour-deposited or a cube having a polarization-sensitive splittingplane. If the beams 5 and 6 are given a slightly different direction, 34and 35 are spatial filters. Such a filter may consist of a telescopicsystem having a pinhole in the focal point, or of pinholes for thedetection systems 13 and 22. Combinations of the above-mentioned threebeam-separating methods are alternatively possible. If the separation ofthe beams reflected by the record carrier and the beams emitted by theradiation sources must be accompanied by a minimal amount of radiationloss, the beam splitter 8 may be replaced. by a polarizing beam spitterand a λ/4 plate.

The detection systems 13 and 22 for generating a focus error signalS_(f), S_(f1), tracking error signals S_(r), S_(r1) and informationsignals S_(i), S_(i1) are only shown diagrammatically in FIG. 1b.Actually, a focus error detection system may include an astigmaticelement, for example a cylindrical lens which is arranged in the path ofthe reflected beam, and the radiation-sensitive detection system maycomprise four detection elements arranged in different quadrants. Theso-called astigmatic focus error detection method as described in U.S.Pat. No. 4,023,033 is then used. The focus error signal mayalternatively be obtained by means of the so-called double Foucaultmethod in which a roof prism is arranged in the reflected beam and inwhich four aligned detection elements are used. The Foucault method isdescribed in, for example, U.S. Pat. No. 4,533,826. Instead of a prism,itis alternatively possible to use a grating as described in U.S. Pat.No. 4,665,310.

The system for generating a tracking error signal may comprise a gratinginthe path of the beam for forming three radiation spots on the recordcarrier, and three radiation-sensitive elements for capturing radiationfrom these three radiation spots, as described in U.S. Pat. No.3,876,842.Another method of generating a tracking error signal is theso-called differential or push-pull method described in U.S. Pat. No.4,491,940.

The invention has been described with reference to the embodiment of theapparatus as shown in FIG. 1b, in which the scanning beam issuccessively used as a write beam and as a read beam. Consequently, theapparatus must have four servosystems: a focus and tracking servosystemfor the guide beam and a focus and tracking servosystem for the scanningbeam. In a preferred embodiment the scanning beam is used as a writebeam only, whilethe guide beam is also used as a read beam. Since noactive tracking control is required for the write beam during thewriting process, three servosystems may be sufficient in this apparatus:a focus and tracking system for the guide beam and a focus servosystemfor the scanning beam. In this preferred embodiment it must be possiblefor the guide beam to be adjusted on any desired recording plane. Tothis end the focus servo should include a recording plane selector 29,identical to the selector 26described hereinbefore, which can beswitched on by means of switch 30 for a reading action. It is alsopossible to focus the guide beam on the guideplane by means of therecording plane selector 29. The switch 30 can then be dispensed with.

It is of course possible to use various scanning beams, each with theirownservosystems, in addition to one guide beam in an apparatus. It isthen possible to write, read or erase two or more layers simultaneouslyin accordance with the inventive method. This increases the rate of datatransmission.

Simplified System

FIG. 6 shows a simplified optical information storage system accordingto the invention. The system comprises a multi-layer optical recordcarrier 101 having a stack 102 of three information layers 103, 104 and105, separated by transparent spacer layers 106 and 107. Eachinformation layerreflects at least part of radiation incident on it,whereas layers 103 and 104 transmit at least part of the radiationincident on them. Each information layer has parallel tracks 108 whichare perpendicular to the plane of the drawing, and indicated only forinformation layer 103. Information can be stored as optically readableareas (not shown in FIG. 6) between or in the tracks 108. The readableareas may comprise pits or bumps in the information layer and areas witha reflectivity or a direction of magnetization different from theirsurroundings. The system further comprises a device 110 for scanning theinformation layers. The device comprises a radiation source 111, forexample, a diode laser, generating a radiation beam 112. A beam splitter113, for example a partially transparent mirror, sends the beam towardsfocusing means 114, for instance an objective lens, which focuses thebeam to a scanning spot 115 on one of the information layers 103, 104 or105. Part of the radiation reflected by the information layer istransmitted to a detectionsystem 116 via objective lens 114 and beamsplitter 113. When record carrier 101 is moved with respect to scanningspot 115, in the plane of the information layers, the scanning spot willscan a track on one of the information layers. The radiation beamreflected by the information layer will be modulated by informationstored in the information layer, which modulation can be detected bydetection system 116. The detection system and an electronic processingcircuit 117 convert the modulation in an electric information signalS_(i), representing the information read from the record carrier.Scanning spot 115 may be positioned on a different information layer bychanging the position of the spot along theoptical axis of objectivelens 114, for instance by changing the axial position of the objectivelens or by changing the position of all the optical components of thedevice, i.e the radiation source, beam splitter,objective lens anddetection system.

Device 110 is provided with a focus servosystem in order to keepscanning spot 115 properly focused on an information layer duringmovement of the layer with respect to the spot. A focus error signalS_(f), i.e. a signal representing the axial distance between thescanning spot and an information layer, may be obtained by the so-calledastigmatic method, known from U.S. Pat. No. 4,023,033. To this enddetection system 116 is divided into four quadrants 116a, 116b, 116c and116d, as shown in FIG. 7,each quadrant being connected to processingcircuit 117. Said information signal S_(i) may be derived from the fourquadrants by summing the signals of the quadrants. For a properdetection of the focus error, device 110 introduces astigmatism in theradiation incident on detection system 116, for instance by means ofbeam splitter 113. As a consequence, the shape of the radiation spot ondetection system 116 changes as function of the focus error betweenshapes 118, 119 and 120 as indicated in FIG. 7 for a scanning spot belowthe information layer, in focus on theinformation layer and above theinformation layer, respectively. Focus error signal S_(f) may be derivedby summing the signals from opposite quadrants to two sum signals andforming a difference signal of the two sum signals. Focus error signalS_(f) is used as input for a focus servocircuit 121 which comprisingservo electronics. The output of circuit 121 is used to control a linearmotor 122 which can change the axial position of objective lens 114,thereby influencing the focus error.

Device 110 is also provided with a radial servosystem in order to keepscanning spot 115 on a track of an information layer. A radial errorsignal S_(r), i.e. a signal representing the distance between thecentreof scanning spot 115 and the centre line of a track 108 to bescanned, may be obtained by the so-called two-beam method, known from,for example, U.S. Pat. No. 3,876,842. To this end device 110 is providedwith dividing means, for instance a grating 123, positioned in radiationbeam 112. The grating splits beam 112 into a +1^(st), -1^(st) and 0^(th)-order beam, i.e. a first tracking beam 124, a second tracking beam 125and a main beam 126. For the sake of clarity only the full paths of thefirst tracking beam and of the main beam are shown. The two trackingbeams and the main beam are focused into three spots on the informationlayer to be scanned by the objective system 114, i.e. information layer103 for the situation given in FIG. 6. The shift in FIG. 6 between theposition of first tracking beam 124 and main beam 126 at the location ofobjective lens 114 has been exaggerated for the sake of clarity. FIG. 8shows the positions of the three spots formed on information layer 103for the case where there are no radial tracking errors. The track pitchor track periodis equal to q. Track 127 is the track which the scanningspot must follow at the moment. The first and second tracking beams formtracking spot 128 and tracking spot 129, respectively. Scanning spot 15formed by the main beam 26 is located on track 27. If them is notracking error, as in FIG. 7, the distance between a tracking spot andthe centre line of the track 127 is equal to x₀. The value of x₀ dependson the way in which a radial tracking error signal is derived from theradiation in the tracking beams. Common values are q/4, q/2 and 3q/4. Asis shown in FIG. 6, the radiation of the two tracking beams 124 and 125reflected by the information layer is transmitted to detection systems130 and 131, respectively, via objective lens 114 and beam splitter 113.Detection systems 130 and 131 are connected to processing circuit 117,which derivesradial error signal S_(r) by subtracting the two signalsfrom detection systems 130 and 131. Radial error signal S_(r) is used asinput for a radial servo circuit 132 which comprising servo electronics.The output ofcircuit 132 is used to control linear motor 122 which canalso change the transverse position of objective lens 114, therebyinfluencing the radial error.

In general, the presence of information layers close to the informationlayer on which the main beam 126 is focused will affect focus errorsignalS_(f) in the form of crosstalk, as will be explained withreference to FIG. 9. This figure shows focus error signal S_(f) as afunction of the axial displacement z of scanning spot 115. Curve 135 isthe so-called S-curve due to information layer 103. It shows two extrema136 and 137 anda zero-crossing 138 between the extrema. When thescanning spot is at the axial position indicated by zero-crossing 38,the scanning spot is in the plane of information layer 103. Duringscanning of this layer, the focus servosystem, comprising detectionsystem 116, processing circuit 117, focus servo circuit 121 and thelinear motor 122, will try to keep the scanning spot at zero-crossing138. At the same time, neighbouring information layer 4 also generates afocus error signal with an S-curve 139, causing crosstalk on S-curve135. The total focus error signal is thesum of the S-curves of thevarious information layers. S-curve 139 causes an offset atzero-crossing 138 of S-curve 135, resulting in a zero-crossing of thetotal focus error signal at a z-position different from the position ofzero-crossing 138. In the example of the figure, scanning spot 115 willthen not be located at information layer 103, but at a positionintermediate between information layers 103 and 104. The offset can beavoided by increasing the spacing between information layers103 and 104,i.e. by increasing the distance between S-curves 135 and 139. Theproximity of the S-curves also causes asymmetry in the shape of thecurves, which might result in capturing problems of the focusservosystem.For known devices the spacing between the S-curves mustpreferably be takenas at least 4 times the peak-to-peak distance S_(p)of an S-curve to avoid said offset and asymmetry. The related minimumdistance between the information layers is 4 n S_(p), with n therefractive index of spacer layer 106 between the two information layers.The maximum distance is preferably 8 n S_(p) in order to have a highinformation density of the record carrier.

In a special embodiment of the device according to the invention theminimum distance between the information layers may be further reducedby reducing the crosstalk of the focus error signals. The crosstalk isdue toradiation from currently unscanned information planes and incidenton detection system 116, as has been explained in the previousparagraph. Since the currently unscanned information planes do not liein the plane of the scanning spot 115, the radiation from these planeswill not be in focus on detection system 116, and therefore form arelatively large radiation spot on the radiation-sensitive surface ofthe detection system.The radiation-sensitive surface of detection system116 is bounded by the outer rectangle of element 116 in FIG. 7. In thespecial embodiment of thedevice the size of the radiation-sensitivesurface is made smaller than in known devices. The largest dimension ofthe radiation-sensitive surface ispreferably smaller than 3 times thediameter of the radiation spot formed on the surface when the radiationbeam is optimally focused on an information layer. Then the influence ofradiation from unscanned information layers is relatively small, and,consequently, also the crosstalk. The largest dimension of theradiation-sensitive surface is preferably larger than 1.5 times thediameter of the radiation spot, because smaller dimensions will causepart of the radiation in shapes 118 and 120 in FIG. 7 to fall outsidethe radiation-sensitive surface, therebyreducing the magnitude of thefocus error signal as well as the value of S_(p). For detection system116 in FIG. 7, this means that the length ofthe diagonal of the squareradiation-sensitive surface is preferably in therange between 1.5 and 3times the diameter of shape 119. A spot diameter of30 μm results in adiagonal range from 45 to 90 μm with a preferred value of 60 μm.

The effect of the small detection system 116 on the S-curves isindicated by the broken lines in FIG. 9. The total width of each S-curveis reduced to less than twice the length of S_(p), and thereby also thecrosstalk of S-curve 139 on S-curve 135. The minimum distance ofinformation layers can now be reduced to 1.5 n S_(p). The maximumdistance is preferably 4 n S_(p). A preferred distance within the rangeis 2 n S_(p). When the peak-to-peak distance is equal to 12 μm and therefractive index of thespacer layers is 1.56, the thickness of thespacer layer is preferably between 28 μm and 75 μm, with a preferredvalue of 37 μm. A focusservosystem specially designed for scanningmulti-layer record carriers hasa peak-m-peak distance of 8 μm. This isachieved by increasing the numerical aperture of the radiation beamincident on detection system 116 and introducing an appropriate mount ofastigmatism in this beam. Said 8 μm peak-to-peak distance and arefractive index of 1.56 give a preferred thickness range from 19 μm to50 μm. A small focus offset due to crosstalk at the lower end of theranges may be compensated by an electronic offset. The value of theelectronic offset preferably depends on the thickness of the spacerlayers and the reflectivity of the information layers.

Although the above discussion is based on a focus servosystem accordingto the astigmatic method, the invention is not limited to this method.It canbe used in each system comprising a focus servosystem having anS-curve. Examples of such servosystems are disclosed in U.S. Pat. No.4,533,826 using the Foucault or knife-edge method and in Japanese PatentApplicationno. 60-217 535 using the beam-size method.

The proximity of unscanned information layers close to the scanning spotalso causes crosstalk in the radial error signal S_(r), which arises inabout the same way as the crosstalk in the focus error signal S_(f).Radiation of the two trackingbeams 124, 125 reflected from the currentlyscanned information layer gives a reasonably small radiation spot oneach of the detection systems 130 and 131. Radiation reflected fromcurrently unscanned information layers gives a relatively largeradiation spot on each detection system. The size of detection systems130 and 131 should therefore be reduced as far as possible. The lengthof the diagonal of theradiation-sensitive surface of each of thedetection systems preferably ranges between 1 and 3 times the diameterof the radiation spot on the surface from a tracking beam optimallyfocused on an information layer.

Radiation of main beam 126 reflected off currently unscanned informationlayers forms a radiation spot centred on detection system 116. Dependingon the thickness of the spacer layers which determines the size of thespot, radiation of the spot might fall on detection systems 130 and 131.Since the intensity of the main beam is generally substantially greaterthan the intensity of the tracking beams, the crosstalk of radiationfrom the main beam on the radial error signal may be considerable. Thecrosstalk is reduced according to the invention by choosing the power inthe main beam to be smaller than six times the power in each of thetracking beams. An improved reduction is obtained if the power in themainbeam is smaller than four times the power in each of the trackingbeams. The reduced power in the main beam is sufficient to write anderase information in most types of information layers.

The above two measures for reducing crosstalk between radial errorsignals are independent of the method used to generate the radial errorsignal. Examples of methods in which one or both of the above measurescan be applied are the one-beam push-pull method as disclosed in U.S.Pat. No. 4,057,833, the two-beam push-pull method as disclosed inEuropean Patent Application no. 0 201 603 and the three-beam method asdisclosed in U.S. Pat. No. 3,376,842.

Spherical aberration incurred by main beam 126 in traversing material ofrecord carrier 101 may be compensated by introducing sphericalaberration of a different sign in the beam. Objective lens 114 mayfunction as compensator by designing the lens such that it introducesthe amount of spherical aberration in the beam required for a certainheight of scanningspot 115 in the record carrier. Such an objective lensis known from the European Patent no. 0 146 178 (U.S. Pat. No.4,668,056). When the height of the scanning spot deviates from theheight for which the spherical aberration is compensated, additional,uncompensated spherical aberration is introduced in the main beam. Itgives rise to a reduced quality of scanning spot 115 which can beexpressed by a decrease r of the Strehl intensity of the scanning spot.The additional aberration is an odd function of the height deviation,resulting in a scanning spot having a different intensity distributionfor an equal positive and negative heightdeviation. In spite of thesedifferent intensity distributions, the qualityof the information signalS_(i) turns out to depend on the inverse squarevalue of the aberration.An equal positive and negative height deviation thus give about the samereduction in quality of the information signal. Hence, there exists arange of thicknesses with only a small reduction of the quality of theinformation signal, which range is located symmetrically around thethickness for which the radiation beam is compensated. The extent of therange is determined by the minimum quality of the information signal asrequired by the system. The extent can also be expressed in terms of themaximally permissible decrease r of the Strehl intensity due tospherical aberration. In general the designer of ascanning device willhave a tolerance budget for the decrease of the Strehlintensity due toall optical aberrations. Part of the budget will be allocated to adecrease caused by spherical aberration. This part determines the extentof said range.

According to the invention stack 102 of information layers is locatedwithin the range of thicknesses with a permissible reduction of thequality of the information signal. When the heights of the outermostinformation layers of the stack are within the range of thicknesses, allinformation layers can be scanned with a well corrected scanning spot.In order to make optimum use of the range, such an mount of sphericalaberration is introduced in the main beam that the scanning spot issubstantially free from spherical aberration at approximately half theheight of the stack. If the extent of the range, i.e. the maximumthickness of stack 102, is represented by 2d, then ##EQU3##in which n isthe refractive index of the spacer layers, λ is the vacuum wavelength ofthe radiation beam and NA is the numerical aperture of objective lens114. If the system has the following parameter values: λ=780 nm, n=1.56,NA=0.52 and r=0.05, then 2d=215 μm. Hence, theheight of the stack ofinformation layers of the system can be up to 215 μm. All informationlayers in the stack can then be scanned by the device with asufficiently corrected scanning spot. If the device has a peak-to-peakdistance of the S-curve of 12 μm with an associated optimum distance ofthe information layers of 37 μm, then the stack cancontain fiveinformation layers. It also turns out that the additional sphericalaberration in the radiation beam within the extent of the range does notinfluence the focusing properties of the focus servo system. Hence thescanning spot can be focused properly on any information layer withinthe range without taking additional measures.

FIG. 10 shows a record carrier 140 comprising a transparent substrate141. The substrate has on one side an entrance face 142 on which mainbeam 126 is incident and on the other side stack 102 comprisinginformation layers 103, 104 and 105. The main beam is corrected for thespherical aberration introduced by a substrate having a nominalthickness and by half the thickness of stack 102. The additionalspherical aberration due to thickness variations of the substratereduces the maximum allowable thickness of the stack. For the above setof parameter values, a peak-to-peak thickness tolerance of 100 μm of thesubstrate and a refractive index of 1.56 of the material of thesubstrate, the maximum thickness of the stack is given by 215-100=115μm. This stack can contain three instead of five information layers.Another example of a system according to the invention with a recordcarrier as shown in FIG. 10, which system is less tolerant for sphericalaberration, has parameter values: λ=635 nm, n=1.56, NA=0.52, S_(p) =8μm, r=0.01 and a thickness tolerance of the substrate of 40 μm. Theextent 2d of the range is then equal to 78 μm, and the maximum thicknessof the stack ofthe record carrier is 78-40=38 μm. The optimum spacing 2n S_(p) of the information layers in the stack is equal to 25 μm.Therefore, the stack can contain two information layers. To furtherreduce the crosstalk due to the proximity of the layers, the spacing maybe increased to 38 μm without affecting the maximum amount ofinformation to be stored in the record carrier. The information layersof such a record carrier may bescanned by a device with a single, fixedcompensation of the spherical aberration.

The refractive index of one or more spacer layers may be equal to one,i.e.the spacer is an air layer. As an example, spacer layer 106 in FIG.10 may have a refractive index of one. The record carrier 140 thencomprises substrate 142 with information layer 103 being a, forinstance, embossed surface of the substrate, and information layers 104and 105 being two surfaces of a plate constituting spacer layer 107. Theplate and the substrate are kept at the required distance by means ofspacer rings, which are not shown in the figure. From the above formulait is clear thatthe air spacer does not contribute to the sphericalaberration. Hence, the maximum thickness of stack 102 is then equal tothe sum of the value of 2das calculated from the formula using therefractive index of spacer layer 107 and the thickness of the spacerlayer 106, reduced by the tolerance onthe thickness of the substrate ifnecessary.

If the refractive index of the substrate is not equal to that of thespacerlayers, the maximum additional spherical aberration of thesubstrate must be calculated first and the resulting reduction of theStrehl intensity must be subtracted from the maximum allowed reductiondue to spherical aberration. The remaining reduction can then be used tocalculate the maximum thickness of the stack.

The record carrier may be provided with several stacks of informationlayers. A device for scanning these layers should be provided with anadjustable spherical aberration compensator, which compensator requiresonly one setting of the compensation for each stack of layers.

The feature of scanning a stack of information layers with a singlespherical aberration compensation may be advantageously combined withthe feature of the minimum distance of the information layers, therebyproviding a high-density record carrier and a relatively low costscanningdevice. However, the application is not limited to thecombination of the two features. As an example a system using said firstfeature and not saidsecond feature scans the record carrier with tworadiation beams. The scanning spot of a first beam is guided by aninformation layer, whereas the position of the scanning spot of a secondbeam is coupled to the firstscanning spot and scans one or more otherinformation layers. There is no crosstalk between servo error signals,and the distance of the informationlayers can be reduced accordingly.The height of the stack is limited by the spherical aberration incurredby the second beam according to said first feature. An example of asystem using said second feature and not said first feature comprises aspherical aberration compensator providing a separate compensation foreach information layer. The minimum distance between the informationlayers is determined by the crosstalk on error signals for focus and/ortracking servo systems, whereas there is no requirement on the maximumheight of the stack of information layers.

What is claimed is:
 1. An information storage system comprising anoptical record carrier having a stack of information layers alternatingwith spacer layers,and a device for scanning the information layers,which device is provided with a radiation source and focusing means forselectively focusing a radiation beam from the radiation source on theseparate information layers, characterized in that the focusing meansintroduce such a fixed, stack-associated spherical aberration in theradiation beam that this aberration compensates the spherical aberrationincurred by the radiation beam when it is focused at approximately halfthe height of the stack of information layers.
 2. An information storagesystem as claimed in claim 1, characterized in that said stack ofinformation layers has a height no greater than approximately 100 μm,and said focusing means comprises an objective system having a numericalaperture of approximately 0.52.
 3. An information storage system asclaimed in claim 1, characterized in that said focusing means comprisesa single corrector plate only, placed in an uncollimated part of theradiation beam, for compensating spherical aberration.
 4. An informationstorage system as claimed in claim 1, characterized in that thethickness of the stack is smaller than 2d which is a function of therefractive index of the material of the stack and of the numericalaperture of the focusing means.
 5. An information storage system asclaimed in claim 4, characterized in that 2d is also a function of themaximally admissible decrease of the Strehl intensity of the radiationbeam due to spherical aberration.
 6. An information storage systemcomprising an optical record carrier having a stack of informationlayers alternating with spacer layers,and a device for scanning theinformation layers, which device is provided with a radiation source andfocusing means for selectively focusing a radiation beam from theradiation source on the separate information layers, characterized inthat during reading the focusing means continuously introduces such afixed, stack-associated spherical aberration in the radiation beam thatthis aberration compensates the spherical aberration incurred by theradiation beam when it is focused at approximately half the height ofthe stack of information layers.
 7. An information system as claimed inclaim 6, characterized in that said stack of information layers has aheight no greater than approximately 100 μm, and said focusing meanscomprises an objective system having a numerical aperture ofapproximately 0.52.
 8. An information system as claimed in claim 6,characterized in that said focusing means comprises a single correctorplate only, placed in an uncollimated part of the radiation beam, forcompensating spherical aberration.
 9. An information system as claimedin claim 6, characterized in that the thickness of the stack is smallerthan 2d which is a function of the refractive index of the material ofthe stack and of the numerical aperture of the focusing means.
 10. Aninformation system as claimed in claim 9, characterized in that 2d isalso a function of the maximally admissible decrease of the Strehlintensity of the radiation beam due to spherical aberration.
 11. Aninformation storage system comprising an optical record carrier having astack of information layers alternating with spacer layers,and a devicefor scanning the information layers, which device is provided with aradiation source and focusing means for selectively focusing a radiationbeam from the radiation source on the separate information layers,characterized in that:the thickness of the stack is smaller than 2dwhich is a function of the refractive index of the material of thestack, of the numerical aperture of the focusing means, and of themaximally admissible decrease of the Strehl intensity of the radiationbeam due to spherical aberration, the value 2d is defined by theequation ##EQU4## in which n is the refractive index of the spacerlayers, λ is the vacuum wavelength of the radiation beam, NA is thenumerical aperture of the focusing means, and r is the maximallyadmissible decrease of the Strehl intensity due to spherical aberration,and the focusing means introduces such a fixed, stack-associatedspherical aberration in the radiation beam that this aberrationcompensates the spherical aberration incurred by the radiation beam whenit is focused at approximately half the height of the stack ofinformation layers.
 12. An information storage system as claimed inclaim 11, in which the record carrier comprises a transparent substratehaving an entrance face for the radiation beam at one side and saidstack at the other side, characterized in that the thickness of thestack is smaller than 2d minus the thickness tolerance of the substrate.13. An information storage system as claimed in claim 11, wherein r isequal to 0.05.
 14. An information storage system as claimed in claim 11,wherein r is equal to 0.01.